JP-A-2002-260827 discloses a conventionally known glow plug. As shown in FIG. 11, the glow plug includes a cylindrical housing 90, a heater 91 that is fixed in the housing 90 and includes a heat generating portion 91a protruding from the front end of the housing 90, a rod-like center shaft 92 which is disposed in the housing 90 and includes the rear end portion protruding from the rear end of the housing 90, and a pin terminal 93 that is fitted to the rear end portion of the center shaft 92 and is used to supply electricity to the heat generating portion 91a from the outside through the center shaft 92.
The heater 91 is a ceramic heater which is fitted into an outer cylinder 94 fitted to the front end of the housing 90 and made of a conductive metal and includes the heat generating portion 91a protruding from the front end of the outer cylinder 94 and the housing 90.
The ceramic heater 91 includes a rod-like insulating body 80 that contains Si3N4 as a main component, a U-shaped heat generating body 81 that is embedded in the front end portion of the insulating body 80 and contains WC as a main component, a first lead wire 82 having one end connected to one end of the heat generating body 81 and the other end exposed to a part of the outer peripheral surface of the insulating body 80, and a second lead wire 83 having one end connected to the other end of the heat generating body 81 and the other end exposed to the other part of the outer peripheral surface of the insulating body 80. The other end of the first lead wire 82 itself is connected to the outer cylinder 94, the other end of the second lead wire 83 itself is connected to a current-carrying coil 95, and the current-carrying coil 95 is connected to the center shaft 92. The front end portion of the insulating body 80, in which the heat generating body 81 is embedded, is the heat generating portion 91a of the ceramic heater 91.
The pin terminal 93 includes a small-diameter portion 93a that is formed at the rear end portion thereof and a large-diameter portion 93b that is formed at the front end portion thereof. The rear end surface of the small-diameter portion 93a has the shape of a curved surface. A first flange 93c, which comes into contact with an insulator 96 provided at the rear end portion of the housing 90 and made of an insulating material, is formed at the front end portion of the large-diameter portion 93b. Further, a second flange 93d is formed at the rear end portion of the large-diameter portion 93b, and an engagement portion 93e to which an engagement protrusion 85a of a cap 97 making current flow from the outside is locked is formed between the first and second flanges 93c and 93d. 
When male threads 90a of the housing 90 are engaged with a cylinder head of a diesel engine, the heat generating portion 91a of the ceramic heater 91 is positioned in the combustion chamber of the diesel engine. Further, the housing 90 is grounded to the cylinder head, and the cap 97 connected to a battery is fitted to the pin terminal 93. The cap 97 includes a cup-shaped cap main body 85 and a conductive member 87. The conductive member 87 is fixed by a fixing member 86, which is provided inner side of the cap main body 85, so as to extend from the inner side toward an open portion. An engagement protrusion 85a, which protrudes inward, is formed on the inner portion of the cap main body 85 close to the open portion. The conductive member 87 is connected to a lead wire 88 that is connected to a battery.
When the cap 97 is fitted to the pin terminal 93, the engagement protrusion 85a of the cap main body 85 gets over the second flange 93d and is engaged with the engagement portion 93e. In this state, the conductive member 87 comes into contact with the surface of the small-diameter portion 93a. 
Accordingly, a voltage is applied between the housing 90, the outer cylinder 94 and the first lead wire 82, the pin terminal 93, the center shaft 92, and the current-carrying coil 95 and the second lead wire 83, so that the heat generating portion 91a of the ceramic heater 91 generates heat by the heat generating body 81. In so doing, the diesel engine starts.
However, the pin terminal in the related art includes the first flange formed at the front end portion of the large-diameter portion and the second flange formed at the rear end of the large-diameter portion. Accordingly, in the pin terminal, the crimp formation region, which is formed so as to have the even outer diameter and be capable of being crimped, is limited between the first and second flanges and is short. For this reason, since it is difficult to solidly join the center shaft to the pin terminal by the short crimp formation region, the joint strength between the center shaft and the pin terminal should be ensured by employing a slightly long center shaft. For this reason, in the pin terminal, the reduction in the amount of material has been insufficient and the reduction in weight has been insufficient. Further, since the glow plug also employs the pin terminal and employs a slightly long center shaft, the reduction in the amount of material is insufficient and the reduction in weight is insufficient.